The invention relates to porous, powdery polypropylene having a large internal surface accessible from the exterior and to processes for its production, as well as its utilization.
It is known how to produce polypropylene with a porous structure, particularly in the shape of compact structures such as sheets, but also in the shape of films, filaments and hollow filaments. U.S. Pat. No. 3,607,793, for instance, describe a process by which, proceeding from small polypropylene particles of a size from approximately 0.02 to 0.5 .mu.m and dissolving these in hydrocarbons, porous films may be obtained if cooling is performed at a rate of less than 100.degree. C. per minute.
This process, however, is not usable for the production of powdery, porous polypropylene. Considerable difficulties will arise, if transformation of porous structures obtained by the doctrine of U.S. Pat. No. 3,607,793 into powdery state is attempted by size reduction. On being ground, for instance, the material will, respectively, become viscous or begin to melt. A perfect porous, powder-like product can also not be obtained by intensive cooling, since the structures will be crunched and ruptured during the grinding process. Also, no powder with favorable particle size distribution can be obtained in this manner.
Also, compact masses of polypropylene with porous structure, as described in DE published application 2 737 745, can be pulverrized in this manner only under difficulties and will yield a powder having unsatisfactory properties in many respects.
Size reduction under liquid nitrogen will still not yield powders with fully satisfactory properties. Firstly, operating with liquid nitrogen is costly, as it is energy-intensive and the generation of gas in large quantities caused by evaporating nitrogen is noisome. Finally, the formation of fibrous edges at the cut or broken surfaces of the particulates, exerting an unfavorable effect upon the properties of the powder, e.g. its free-flowing qualities, cannot be avoided even by such intensive cooling as obtainable by using liquid nitrogen.
Porous, powdery substances may be used in the most different fields, e.g. in adsorption, as additives or fillers, or the like. The need exists therefore, for such porous substances, particularly those with improved properties, and also for appropriate production processes.